A. Field of the Invention
The present invention relates to a method of forming a protective film that is a component of a magnetic recording medium, and in particular to a method of forming an extremely thin protective film. The invention further relates to a protective film obtained by such a method. The invention still further relates to a magnetic recording medium including such a protective film.
B. Description of the Related Art
A magnetic recording medium is obtained generally by forming a magnetic recording layer and other layers on a substrate, and by further forming a protective film thereon. A protective film works as a slide-resistant member and/or a wear-resistant member. Various types of protective films have been disclosed.
A protective film needs to have abilities to hinder damage by contact or sliding of a magnetic head with a magnetic recording layer and to avoid corrosion of the magnetic recording layer.
One of representative protective films satisfying these requirements is a diamond-like carbon (DLC) film using carbon. A DLC film exhibits good surface smoothness and a large hardness, and thus, has a good surface covering performance that must be borne by a protective film. A DLC film is generally formed on a magnetic recording layer by means of a sputtering method, a plasma CVD method, or the like.
Meanwhile, a recording system of a magnetic recording medium has been changing from a longitudinal system to a perpendicular system, the latter being capable of higher density recording. The perpendicular system has been studied extensively for further raising the recording density. In this context, a protective film needs to be made as thin as possible while holding the performances of durability and corrosion resistance.
Several methods of forming a protective film have been disclosed. Japanese Unexamined Patent Application Publication No. 2003-303410 discloses a magnetic disk having at least a magnetic metal film, a carbon protective film, and a fluorine-containing lubrication film provided on a non-magnetic substrate. The carbon protective film consists of a diamond-like carbon film having a thickness not larger than 4 nm, and the ratio of infrared absorption intensity caused by a C—H bond to infrared absorption intensity caused by a C—F bond of the lubrication film in the vicinity of 2,920 cm-1, as measured by a FT-IR using an ATR (attenuated total reflectance) method, is in the range of 0.035 to 0.060.
Japanese Unexamined Patent Application Publication No. 2007-265586 discloses a method of manufacturing a magnetic disk provided with a magnetic layer, a protective layer, and a lubricant layer in this order on a substrate. In this method, after the magnetic layer and the protective layer are formed in this order on the substrate, the protective layer is exposed to plasma under normal pressure and then the lubricant layer is formed on the protective layer. A plasma treatment is conducted in this particular method on a hydrogenated carbon protective layer having a thickness in the range of 0.5 to 3.0 nm formed by a plasma CVD method. The plasma treatment is carried out in at least one gas selected from a nitrogen gas, an argon gas, an oxygen gas, and a fluorine-containing hydrocarbon gas. It is asserted that the surface of the protective layer is favorably reformed and the affinity with the lubricant layer is enhanced.
These conventional techniques, however, fail in suppressing cobalt elution out of the magnetic recording layer, and hardly obtain an extremely thin protective film, specifically with a thickness not larger than 3 nm.
The present invention is directed to overcoming or at least reducing the effects of one or more of the problems set forth above.